Klebold, Malvo and Dzhokhar Tsarnaev all might be seen as more passive teenagers who were attached to more assertive mentors whose actions had deadly consequences, according to mental health experts.

On April 20, 1999, Columbine High School 18-year-olds Harris and Klebold slaughtered 12 students and one teacher in a 45-minute rampage before killing themselves. Post-mortem, experts diagnosed Harris as a psychopath.

During three weeks in October 2002, military veteran Muhammad and 17-year-old Malvo killed 10 people on a rampage throughout Washington, D.C., Maryland and Virginia. Muhammad, at 41 and a father figure to the teen, had planned to train youths to carry out mass shootings. He was executed by lethal injection in 2003. Malvo is serving six life sentences in prison.

"Very often, these are pairs of perpetrators, one dominant figure who has psychopathic tendencies and the other a dependent figure," said Dr. Igor Galynker, director of the Family Center for Bipolar in the psychiatry department at Beth Israel Medical Center in New York City.

"It's not accidental," he said of the attraction to the stronger figure. "But it's almost opportunistic in the negative."

Attachment issues and the anatomy of the immature teenage brain can create a killer, according to Galynker.

"In terms attachment, Harris was psychologically a classic psychopath," he said. "And Klebold was a depressed loner who needed a role model and an attachment figure and acceptance. … Harris happened to drag him into his orbit."

In the case of Dzhokhar Tsarnaev, it may be too early to know, but Galynker said someone with an insecure attachment would "likely become psychologically dependent on someone who accepted them and loved to get close."

Teenage behavior, as every parent knows, is marked by impulsiveness and the inability to see the consequences of actions.

Now, science has confirmed that the human brain is not fully developed until the age of 25. The brain develops from back to front -- from the emotional limbic system first to the rational frontal precortex last.

As authorities try to find answers as to why the Tsarnaev brothers, both well-educated American citizens with talent and promise, turned to terrorism, online forums have debated young Dzhokhar Tsarnaev's culpability.

New Yorker editor David Remnick painted a sympathetic portrait of the brothers.

"I think this had to do with his older brother," he wrote. "Unless he was some sort of sleeper agent, I think his brother had a pretty strong influence. Tamerlan maybe felt like he didn't belong, and he might have brainwashed Dzhokhar into some radical view that twisted things in the Koran."

Columnist John Nolte was outraged in the conservative Breitbart report.

"It is one thing to never forget, no matter the evil with which we are faced, our own humanity," he wrote. "It is quite another, however, and quite dangerous, to forget that we are dealing with evil. Such a thing not only invites more evil; worse still, it is a cruel slap in the face to the victims of evil."

He called Remnick's essay "poorly timed and even shameful attempt to define evil by something other than evil." He argued the brothers "made a choice" that killed four people and maimed more than 100 more.

But psychiatrists like Galynker said that a 19-year-old like Dzhokhar Tsarnaev would have "difficulty with executive functioning, assessing risk and understanding the consequences of behavior." At 26, Tamerlan, despite his alleged terrorist motives, was "fully matured and [his brain] myelinated."

Myelination is the process by which a fatty layer, called myelin, accumulates around nerve cells. It is a key to healthy brain development.